Three motor walkers are, generally, more-capable but more
complex (at least electrically) than 2 motors walkers. Just
a handful of 3 motors designs have been made public, so I'll
pass along what I've managed to pick up.

Motor geometryThere are bound to be a number of ways to hook up 3
motors for a walker; the most-common is a fairly
straight-forward advancement
on 2-motor / 4-leg walkers: